Ultron made his first Marvel appearance in Avengers No. 54, his true identity hidden as Crimson Cowl. Using Marvel’s digital comic reading tool, you can sample his appearances in Avengers No. 55 (pictured), Avengers No. 201 and Avengers No. 202. You can read more about Ultron’s comic book legacy at Wikipedia (spoilers follow):

The character leads the Masters of Evil against the Avengers, having hypnotized Edwin Jarvis into working for him. In the following issue, #55 (Aug. 1968), the character is identified as Ultron-5, the living automaton, although his origin is still unknown.[2] In Avengers 57 – 58 (Oct-Nov. 1968) in aflashback sequence it is revealed that Ultron is the creator of the “synthezoid” the Vision, whom it tries to use as a weapon to destroy the Avengers.

Writer and director Joss Whedon will helm the 2015 sequel. Here’s more about Spader’s career, from the release:

Spader earned three Emmy Awards for his role as Alan Shore on “Boston Legal” and “The Practice,” and will return to television this fall in NBC’s highly anticipated series “The Blacklist.” Most recently Spader was seen in the critically-acclaimed film “Lincoln” directed by Steven Spielberg and has previously worked with a long list of accomplished film directors including David Cronenberg, Curtis Hansen, Sidney Lumet, Mike Nichols, Tim Robbins, Robert Rodriguez, Steven Soderbergh, and Oliver Stone.